An aircraft may include various types of rotating electrical machines such as, for example, generators, motors, and motor/generators. Motor/generators are used as starter-generators in some aircraft, since this type of rotating electrical machine may be operated in both a motor mode and a generator mode. A starter-generator may be used to start the engines or auxiliary power unit (APU) of an aircraft when operating as a motor, and to supply electrical power to the aircraft power distribution system when operating as a generator. Thus, when operating as a motor, a starter-generator may be used to start the engines.
One particular type of aircraft starter-generator includes three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter generator, and a main motor/generator. The PMG includes permanent magnets on its rotor. When the PMG rotor rotates, AC currents are induced in stator windings of the PMG. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current if the starter-generator is operating as a generator. Conversely, if the starter-generator is operating as a motor, the control device supplies AC power.
If the starter-generator is operating in generator mode, DC current from the regulator or control device is supplied to stator windings of the exciter. As the exciter rotor rotates, three phases of AC current are typically induced in the exciter rotor windings. Rectifier circuits that rotate with the exciter rotor rectify this three-phase AC current, and the resulting DC currents are provided to the rotor windings of the main motor/generator. Finally, as the main motor/generator rotor rotates, three phases of AC current are typically induced in the main motor/generator stator, and this three-phase AC output can then be provided to a load.
If the starter-generator is operating motor mode, AC power from the control device is supplied to the exciter stator. This AC power induces, via a transformer effect, an electromagnetic field in the exciter armature, whether the exciter rotor is stationary or rotating. The AC currents produced by this induced field are rectified by the rectifier circuits and supplied to the main motor/generator rotor, which produces a DC field in the rotor. Variable frequency AC power is supplied from the control device to the main motor/generator stator. This AC power produces a rotating magnetic field in the main stator, which causes the main rotor to rotate and supply mechanical output power.
The above-described starter-generator may include relatively complex and heavy power electronics circuits in the control device. For example, some control devices may include inverters, for converting DC to AC power, rectifiers, for converting AC power to DC power, and potentially complex voltage and frequency control circuits. Although brush-type DC machines may alleviate the need for some of these complex and heavy electronic circuits, these also suffer certain drawbacks. For example, the brushes tend to wear fairly quickly, which can reduce machine reliability and increase the need for periodic maintenance and cleaning. Some brush-type DC machines can also suffer what is known as torque ripple during startup. In some instances, the torque ripple can be large, which can result in poor starter performance.
Hence, there is a need for a starter-generator that does not rely on relatively complex and heavy inverters and frequency control circuits for proper operation, and/or does not suffer reduced reliability from brush wear, and/or the need for potentially frequent maintenance and cleaning, and/or does not experience significant torque ripple during startup. The present invention addresses one or more of these needs.